1. Field of the Invention
The present invention relates to a display device, and in particular to a liquid crystal display device, which uses a thin film transistor (TFT) formed on a transparent substrate such as glass or plastics, and a driving method of the same. In addition, the invention relates to an electronic apparatus using the liquid crystal display device.
2. Description of the Related Art
In recent years, with the advance of the communication technology, cellular phones have been widely used. In future, transmission of moving images and transmission of a larger volume of information are expected. On the other hand, through reduction in weight of personal computers, those adapted for mobile communication have been produced. Information terminals called PAD originated in electronic notebooks have also been produced in large quantities and widely used. In addition, with the development of display devices, most of those information portable apparatuses are equipped with a flat panel display.
Moreover, in the recent technique, as a display device provided in them, an active matrix display device tends to be used.
In the active matrix display device, a TFT is arranged for each pixel and a screen is controlled by the TFT. Such an active matrix display device has advantages in that it is high in performance, image quality, and moving image responsiveness, and the like compared with a passive matrix display device. Therefore, it is expected that the main stream of a liquid crystal display device also shifts from passive to active.
In addition, in recent years, among active matrix display devices, manufacturing of a display device using low temperature polysilicon have been promoted. With the low temperature polysilicon, in addition to manufacturing a pixel, a driving circuit can be integrally formed around a pixel portion. Thus, since it is possible to realize compactness and high definition of a display device, the display device is expected to be more widely used in future.
An operation of a pixel portion of an active matrix liquid crystal display device will be hereinafter described. FIG. 3 shows an example of a structure of the active matrix liquid crystal display device. One pixel 302 is constituted by a source signal line S1, a gate signal line G1, a capacitance line C1, a pixel TFT 303, and a storage capacitor 304. However, the capacitance line is not always necessary if other wiring or the like can also be used as the capacitance line. A gate electrode of the pixel TFT 303 is connected to the gate signal line G1. One of a drain region and a source region of the pixel TFT 303 is connected to the source signal line S1, and the other is connected to the storage capacitor 304 and the pixel electrode 305.
A driving method of this pixel will be hereinafter described. When a signal voltage is inputted in the gate signal line G1 and the pixel TFT 303 is turned ON, a signal voltage is inputted from the source signal line S1 and an electric charge is stored in the storage capacitor 304. A voltage is applied to the pixel electrode 305 by this stored electric charge, and the voltage is applied between electrodes sandwiching a liquid crystal. An orientation of molecules of the liquid crystal changes in association with this applied voltage, and a transmitted light amount is controlled.
FIG. 4 shows a relationship between an applied voltage and a transmitted light amount. The applied voltage is changed in the range of −Vm to Vm, whereby the transmitted light amount can be changed. Note that it is assumed that the transmitted light amount reaches a maximum transmitted light amount Tmax when the applied voltage is zero. Here, there is a problem in that, when an electric field is continuously applied in a fixed direction, ions accumulate on one side of a liquid crystal and the liquid crystal deteriorates instantly. Thus, it is a general practice to drive the pixel with an applied voltage of an inverted polarity every time a signal is written in the pixel.
FIG. 5 shows a relationship among a gate signal voltage, a source signal voltage, and a voltage applied to the liquid crystal at the time when this display device is driven. In this figure, an applied voltage to a liquid crystal in certain one pixel is shown with the attention paid to certain one gate signal line and certain one source signal line.
When a gate signal line is selected and a voltage is applied to a liquid crystal, an orientation of liquid crystal molecules changes in accordance with the applied voltage. Consequently, a transmitted light amount changes to display an image. Here, a voltage applied to the liquid crystal changes in the range of −V to V, and its polarity is inverted every time a signal is written in a pixel. Note that |V| is set to a value equal to or lower than |Vm| in FIG. 4.
FIG. 6A shows an example of a sectional view of a pixel portion of a conventional active matrix liquid crystal display device. A pixel TFT 102 and a storage capacitor 103 are formed in a pixel portion 101. Here, reference numeral 104 denotes an insulating substrate of a TFT substrate; 105, a source region or a drain region of the pixel TFT 102; 106, a channel region of the pixel TFT 102; 108, a gate insulating film; and 107 and 112, electrodes of the storage capacitor 103, which sandwich an insulating layer 109 between them. Note that the electrode 107 is formed of a semiconductor layer, and an impurity element is doped in the electrode 107. The electrode 107 is connected to the drain region of the pixel TFT 102. In addition, reference numeral 215 denotes a gate signal line; 210, a source signal line; 116, drain wiring; 113, an interlayer insulating film; 118, a pixel electrode; 119 and 126, orientation films; 120, a liquid crystal; 121, an insulating substrate of an counter substrate; 122, a black matrix (BM); 123, a color filter; 124, a planarization film; and 125, a counter electrode.
In the manufacturing of this active matrix liquid crystal display device, reduction of manufacturing costs and improvement of yield have been advanced by reducing the number of steps therefor.
Here, in order to reduce the number of masks to be used, the pixel electrode 118 to be connected to the drain wiring 116 is directly brought into contact with the drain wiring 116 to achieve conduction.
The source signal line 210 is patterned in the same layer in which the drain wiring 116 and the pixel electrode 118 are patterned. Consequently, a sufficient space part has to be secured between the source signal line 210 and the pixel electrode 118 in order to prevent short-circuit of the source signal line 210 and the pixel electrode 118. In addition, it is necessary to cover this space part with a BM in order to prevent leakage of light from this space part.
FIG. 6B shows a top view of the pixel in this case. For ease of understanding, a part of an area from which the pixel electrode 118 and the BM are removed is shown. Here, FIG. 6A corresponds to a sectional view along line A-A′ in FIG. 6B. Note that, in FIG. 6B, the same reference numerals as those of FIG. 6A denote the same portions. Reference numeral 210 denotes a source signal line; 116, drain wiring; 215, a gate signal line; 118, a pixel electrode; and 220, a semiconductor layer, which is equivalent to reference numerals 105 to 107 in FIG. 6A.
Here, a space part 230 is provided between the source signal line 210 and the pixel electrode 118 to prevent the source signal line 210 and the pixel electrode 118 from short-circuiting. Consequently, the area of the pixel electrode 118 cannot be enlarged. Therefore, an opening ratio cannot be increased. In addition, this space part 230 is covered by a BM 122 provided on an counter substrate in order to prevent leakage of light from this space part 230. Here, it is necessary to arrange the BM 122 to overlap the end of the pixel electrode 118 taking into account deviation at the time when a TFT substrate and the counter substrate are adhered to each other, invasion of light, and the like. There is a problem in that the opening ratio further decreases due to this arrangement.
Consequently, a display device having a structure as shown in FIG. 7A has been proposed. Note that, in FIG. 7A, the same reference numerals as those in FIGS. 6A and 6B denote the same portions.
In FIG. 7A, reference numeral 111 denotes a gate electrode; 114, source wiring; 110, a source signal line; and 115, a gate signal line.
In the display device of the sectional view shown in FIG. 7A, the source signal line 110 is formed simultaneously with the gate electrode 111, and the gate signal line 115 is formed simultaneously with the source wiring 114 and the drain wiring 116. Here, the source signal line 110 is connected to the source region of the pixel TFT 102 by this source wiring 114. With this structure, layers in which a source signal line and a gate signal line are formed can be interchanged without increasing the number of masks. Such an arrangement of the source signal line and the gate signal line is called an inverse-cross structure. With this structure, since the source signal line 110 is arranged in a layer below the drain wiring 116, the pixel electrode 118 can be formed above the source signal line 110 and the opening ratio can be increased.
FIG. 7B shows a top view of FIG. 7A. For ease of understanding, a part of an area from which the pixel electrode 118 and the BM are removed is shown. Here, FIG. 7A corresponds to a sectional view along lines A-A′ and B-B′ in FIG. 7B. The pixel electrode 118 is formed to cover up to the source signal line 110 to prevent leakage of light. Thus, the part of the BM 122 provided on the counter substrate is reduced compared with that in FIG. 6B. In this way, the opening ratio is increased compared with that in FIG. 6.
In the display device using the inverse-cross structure, the gate signal line is formed on the same insulating surface on which the pixel electrode is formed, and an orientation film and a liquid crystal are formed above the insulating surface.
In FIG. 5, it is assumed that a signal voltage for selecting a gate signal line is Vo and a signal voltage for not selecting a gate signal line is set to −Vo. When the number of gate signal lines is assumed to be y, a period in which the gate signal lines are selected (a gate signal line selecting period) is approximately 1/y of one frame period. Thus, the larger y the shorter the gate signal line selecting period becomes, and a ratio of periods in which a signal voltage for not selecting a signal line is applied (a gate signal line non-selecting period) increases. Therefore, the voltage of −Vo continues to be inputted while a pixel is not selected.
In the case in which a standard of a display device is VGA, −Vo is inputted in periods equal to or more than 479/480. A duty at this point is 0.2% or less.
Note that as shown in FIG. 5, since a polarity of a voltage applied to the source signal line is inverted periodically, the voltage does not affect a liquid crystal portion significantly. On the other hand, a voltage inputted in the gate signal line tends to have a fixed polarity as described above. Such a signal voltage inputted in the gate signal line affects a liquid crystal portion arranged immediately above the gate signal line. This becomes a cause for facilitating deterioration of the liquid crystal. In such a case, it is necessary to use a fluorine-based liquid crystal (e.g., T1213, T1216, etc. of Merck & Co., Inc.) with less deterioration, and an inexpensive cyanic liquid crystal cannot be used.